It has been known in the past to drive multiple devices using rotational power supplied to a crankshaft in a motor vehicle. For example, multiple rotating devices, or a reciprocating device such as a cam follower and a rotating device may be both driven from the same crankshaft. This is seen, for example, in tractors for pulling trailers, which may have a reciprocating piston of an air compressor (to operate pneumatic brakes) powered by the crankshaft that also powers an alternator or another pump or compressor.
It would be desirable if power transmission to such separate devices could be independently controlled. For example, there are often situations where one device to be powered (such as an alternator) must be continuously supplied with rotational power, while another such device (such as an air compressor) needs only periodic power. In prior art devices, it is typically necessary to continuously run both devices, i.e., in the example, both the alternator and the air compressor. Even if the air compressor is run in an "unloaded" mode (i.e., not discharging air under pressure), the compressor continuously consumes energy, consumes lubricating oil, and undergoes wear and tear.
It would be desirable if an apparatus could be provided in such situations to permit a device such as the air compressor to be disengaged from the power source while leaving another device such as an alternator engaged with the power source. This would have the benefit of providing better energy efficiency in the motor vehicle (and therefore better fuel economy) by disengaging the device such as the compressor while still leaving the other device such as the alternator operative. Otherwise, in order to disengage the compressor, the other device must also be disengaged, which is obviously undesirable in that it will interfere with proper functioning of the motor vehicle such as a tractor. For example, excessive disengagement of the alternator will cause the batteries to discharge without recharging, creating attendant problems with electrical system components such as lights, ignition, etc.